Ink jet printing technology is used from industrial labeling applications to office printing. Ink jet printers are low cost, low maintenance, quiet printers that provide near laser printing quality with high print speeds. Another advantage of the use of ink jet printers is the development of consistent, high-quality color printing capabilities for the full range of colors. These attributes have made the ink jet printer one of the most popular office printers on the market today. Ink jet printing technology has also expanded into the reprographic market with the advent of large format ink jet plotters, such as the Hewlett Packard Designjet series and Encad's Novajet series. Such plotters can produce a range of sizes of drawings and designs from the standard A size (8.5".times.11") up to an E size (36".times.48") plot. For example, ink jet plotters are used in the production of designs, floor plans, and structures with computer aided design (CAD) software.
Ink jet printing technology is a form of printing that encompasses the projection of ink through a nozzle orifice, forming tiny droplets of a specific diameter, directly onto a substrate, such as paper or film, to form written symbols and drawn images. Several different technologies are used for such projection of ink droplets onto a substrate. For example, in a continuous stream system a continuous stream of ink droplets is discharged from an ink reservoir through a nozzle. The droplets may then be deflected by means of an electrically charged field to the substrate to form an image on an ink-receiving substrate. Those droplets not deflected to the substrate by the electrically charged field are allowed to flow in a straight stream and are collected and recirculated for reuse.
Another example of ink jet printing technology is drop-on-demand systems, a droplet is expelled from an orifice directly to a position on a recording medium in accordance with digital data signals. A droplet is not formed or expelled unless it is to be placed on the recording medium. Since drop-on-demand systems require no ink recovery, charging, or deflection, the system is much simpler than the continuous stream type. There are two types of drop-on-demand ink jet systems.
One type of drop-on-demand system has as its major components an ink filled channel or passageway having a nozzle on one end and a piezoelectric transducer near the other end to produce pressure pulses. The relatively large size of the transducer prevents the close spacing of the nozzles to one another which is necessary for high resolution printing. Additionally, physical limitations of the transducer result in low ink drop velocity. Low drop velocity seriously diminishes tolerances for drop velocity variation and directionality, thus impacting the system's ability to produce high quality copies, and also decreases printing speed. Drop-on-demand systems which use piezoelectric devices to expel the droplets also suffer the disadvantage of a slow printing speed.
The other type of drop-on-demand system is known as thermal ink jet, or bubble jet, and produces high velocity droplets and allows very close spacing of nozzles. The major components of this type of drop-ondemand system are an ink filled channel having a nozzle on one end and a heat generating resistor near the nozzle. Printing signals representing digital information originate an electric current pulse in a resistive layer within each ink passageway near the orifice or nozzle, causing the liquid vehicle of the ink in the immediate vicinity to undergo a liquid-gas phase transition, which creates a bubble. The ink at the orifice is forced out as a propelled droplet as the bubble expands. The drop-on-demand ink jet printers provide simpler, lower cost devices than their continuous stream counterparts, and yet have substantially the same high speed printing capability.
A major problem with inks used in ink jet printers is that the dye colorants contained in the ink tend to fade when exposed to sunlight or artificial light. It is believed that most of the fading of colorants when exposed to light is due to photodegradation mechanisms. These degradation mechanisms include oxidation or reduction of the colorants depending upon the environmental conditions in which the colorant is placed. Fading of a colorant also depends upon the substrate upon which they reside.
Various factors such as temperature, humidity, gaseous reactants, including O.sub.2, O.sub.3, SO.sub.2, and NO.sub.2, and water soluble, nonvolatile photodegradation products have been shown to influence fading of colorants. The factors that effect colorant fading appear to exhibit a certain amount of interdependence. It is due to this complex behavior that observations for the fading of a particular colorant on a particular substrate cannot be applied to colorants and substrates in general.
Under conditions of constant temperature it has been observed that an increase in the relative humidity of the atmosphere increases the fading of a colorant for a variety of colorant-substrate systems (e.g., McLaren, K., J. Soc. Dyers Colour, 1956, 72, 527). For example, as the relative humidity of the atmosphere increases, a fiber may swell because the moisture content of the fiber increases. This aids diffusion of gaseous reactants through the substrate structure.
The ability of a light source to cause photochemical change in a colorant is also dependent upon the spectral distribution of the light source, in particular the proportion of radiation of wavelengths most effective in causing a change in the colorant and the quantum yield of colorant degradation as a function of wavelength. On the basis of photochemical principles, it would be expected that light of higher energy (short wavelengths) would be more effective at causing fading than light of lower energy (long wavelengths). Studies have revealed that this is not always the case. Over 100 colorants of different classes were studied, and it was found that generally the most unstable were faded more efficiently by visible light while those of higher lightfastness were degraded mainly by ultraviolet light (McLaren, K., J. Soc. Dyers Colour, 1956, 72, 86).
The influence of a substrate on colorant stability can be extremely important. Colorant fading may be retarded or promoted by some chemical group within the substrate. Such a group can be a ground-state species or an excited-state species. The porosity of the substrate is also an important factor in colorant stability. A high porosity can promote fading of a colorant by facilitating penetration of moisture and gaseous reactants into the substrate. A substrate may also act as a protective agent by screening the colorant from light of wavelengths capable of causing degradation.
The purity of the substrate is also an important consideration whenever the photochemistry of dyed technical polymers is considered. For example, technical-grade cotton, viscose rayon, polyethylene, polypropylene, and polyisoprene are known to contain carbonyl group impurities. These impurities absorb light of wavelengths greater than 300 nm, which are present in sunlight, and so, excitation of these impurities may lead to reactive species capable of causing colorant fading (van Beek, H. C. A., Col. Res. Appl., 1983, 8(3), 176).
Therefore, for all of these reasons, there exists a great need for methods and compositions which are capable of stabilizing a wide variety of colorants from the effects of both sunlight and artificial light. There is a particular need for an improved ink for ink jet printers that are light-stable.